Support for cmake mode RelWithDebInfo

Two commits in this branch:
- Set up the new mode RelWithDebInfo adding the -g and -O2
- Change Release mode to use -O3

Important! if no mode was supplied to the cmake script, it is going to default to Release mode. We should think if this is the desired approach. If it is, we should made jenkins to pass the RelWithDebInfo to cmake.

The branch as it is now is setting up the optimization and debug levels agreed with Nate and Brian.

I have manually tested the cmake modes and things seems ok. About when to applying this patch, I would go for doing it just after release gazebo 1.3 so all the error that could happen start in a fresh develpment cycle.